In the hydraulically operated suspension systems that suspend an auxiliary axle from a motor vehicle in a deployable manner to help support the vehicle, it is common practice to employ a pair of hydraulically operated actuators. Wherein the actuators are operable to position the auxiliary axle in a stowed condition on the vehicle and in an active condition wherein the auxiliary axle is forced to help support the vehicle with the auxiliary axle wheels. And it is desirable that the actuators be capable of allowing significant up and down travel of the auxiliary axle in a sprung manner relative to the vehicle to accommodate differences in road elevation between the auxiliary axle and the other axles supporting the vehicle. Including when the auxiliary axle wheels encounter bumps and dips in the road surface. And when the auxiliary axle is being used in a minor or major supporting manner.
This springing effect can be provided by employing a gas spring with each of the actuators such that the actuators are allowed to extend and contract with gas spring action and thereby allow the auxiliary axle to adapt in a sprung manner in accommodating differences in road grade between the auxiliary axle and the other axles supporting the vehicle and this includes the auxiliary axle wheel encountering bumps and dips and all while the auxiliary axle continues to help support the vehicle. And it is desirable that this be accomplished without incurring significant gas and hydraulic pressure fluctuation that could adversely affect the desired operation. Including when the actuators are employed in a manner that contributes to a significant degree to vehicle stability and in preventing shock forces on the auxiliary axle wheels from being transmitted to the vehicle such as when the auxiliary axle wheels encounter bumps and dips.
To prevent such adverse effects from occurring in the case of an auxiliary axle that operates at a substantial distance behind the vehicle and is commonly referred to as a trailing axle, each of the actuators may for example be provided with a gas spring wherein the gas springs share a gas pressure accumulator and the gas circuit is charged with a suitable gas at a pressure sufficient to provide the gas springs with a spring compliance meeting the projected demands that can include allowing the trailing axle a wide range of travel to adjust to road grade changes at the trailing axle. However, it may not be practical or desirable to rely on such gas springs with a shared gas pressure accumulator or separate gas pressure accumulators and especially where the actuators are unusually long in extent and are required to extend and retract to a large extent in allowing for the trailing axle to adapt to large road grade changes at the trailing axle as well as when the trailing axle wheels encounter bumps and dips in the road. And this also applies to an auxiliary axle that may be deployed at a different location with respect to the vehicle and it is desired to have similar operating conditions.